To produce a stainless steel sheet by a continuous casting method, a conventional production method comprises the steps of casting a slab having a thickness of at least 100 mm while oscillating a mold in a casting direction, treating the surface of the obtained slab, heating the slab to a temperature not lower than 1,000.degree. C. in a heating furnace, hot rolling the slab by a hot strip mill comprising a rough rolling mill and a finish rolling mill and obtaining a hot strip having a thickness of several millimeters.
When the hot strip thus obtained is cold rolled, to secure the shape (flatness), mechanical and surface properties required for the final product, hot rolled sheet annealing is carried out to soften a strongly hot worked hot strip and scale, etc., on the surface is removed therefrom by grinding after pickling.
The conventional process described above requires a great deal of energy for heating and working the materials in a long and large hot rolling facility, and also cannot be said to be a really excellent production process from the aspect of producibility. In the final product, there are many limitations for using, because a texture develops in the product, and therefore, when a user conducts press working, etc., of the product sheet, anisotropy must be taken into account.
To solve the problem that a long and large hot rolling facility and a great deal of energy and rolling power are necessary for rolling a slab having a thickness of at least 100 mm to a hot strip, studies have been carried out in recent years in search of the process which obtains a slab (thin strip) having a thickness equal or approximate to a hot strip during the continuous casting process.
For examples, feature articles in "Iron and Steels", '85, A197-A256 and "CAMP ISIJ", Vol. 1, 1988, 1674-1705, disclose a process for directly obtaining the hot strip by continuous casting.
In such a continuous casting process, the use of a twin drum system is examined when the gauge of the obtained slab (strip) is at a level of 1 to 10 mm and a use of a twin belt system is examined when the gauge of that is at a level of 20 to 50 mm.
The continuous casting process of this kind produces a slab having a shape approximate to the final shape, and omits or reduces intermediate steps such as a hot rolling step, a heat-treatment step, and so forth. Accordingly, it is known that a structure of the slab greatly affects the mechanical and surface properties of the final product.
Conventionally, very small corrugations of from about 0.2 to 0.1 .mu.m, which are referred to as "roping", have often occurred on the surface of the steel sheet as cold rolled, in the austenitic stainless steel sheet produced by the strip casting process described above. Japanese Unexamined Patent Publication (Kokai) No. 2-19426 teaches to first carry out preliminary cold rolling at a reduction ratio of up to 60%, then to carry out intermediate annealing and to thereafter carry out cold rolling to the thickness of the final product, or a so-called "twice cold rolling method", in cold rolling of the cast-strip so as to reduce this "roping". According to this method, plastic working is applied to the strip consisting of coarse .gamma. grains so as to first flatten mechanically the surface of the strip and at the same time, strain induced by distortion is built up in the internal structure so as to promote the progress of recrystallization during intermediate annealing. After the crystal grains are made smaller and the strain is sufficiently released, the surface corrugation is reduced by the second cold rolling operation on the basis of the same principle. Accordingly, this method teaches that both in the preliminary rolling process and the second cold rolling process, a greater effect can be obtained by rolling at a higher reduction, and if a preliminary rolling reduction is less than 30%, the effect is small.
The cause for the occurrence of roping described above has not yet been clarified sufficiently, but the inventors of the present invention have assumed the reasons as follows. Namely, the .gamma. phase works harden with rolling deformation and at the same time, a rolling texture having a {110}&lt;112&gt; orientation as a primary orientation is formed, so that fine corrugations are induced due to plastic anisotropy of the cold rolled band structure (which resists deformation in the direction of thickness of the sheet) comprising this hardened orientation. For this reason, when a large number of martensite phases (.alpha.' phase) induced by working are generated by increasing the cold rolling reduction, the effect of cutting off the gamma phase occurs and the amount of roping is expected to fall. It is appreciated that Japanese Unexamined Patent Publication (Kokai) No. 2-19426 aims at eventually utilizing this effect.
Japanese Unexamined Patent Publication (Kokai) No. 3-42151 attempts to reduce roping by increasing the amount of a martensite phase generated during cold rolling by setting an Md.sub.30 point calculated from a composition in the range of 30.degree. to 60.degree. C.
The present inventors have examined in detail press workability of the sheet produced by an austenitic stainless steel sheet production process based on strip casting, and have found out that surface defects referred to as "work-surface roughening", which is obviously different from roping occurring during cold rolling and which exhibits a ridge height of at least about 2 .mu.m as described below, in the case that a work manufacturer, etc., carries out press working the product sheet after final annealing in order to produce a final work product. Roping is believed to occur on the basis of the cold rolled structure in which the .gamma. phase and the .alpha.' phase exist in mixture, but work-surface roughening is expected to occur on the basis of the gamma phase subjected to the recrystallization annealing treatment and to comprise entirely different orientations from the cold rolled state from the aspect of the texture, as well. In other works, this work-surface roughening cannot at all be prevented by the roping solution methods described in the prior art references described above.
In other words, work-surface roughening remarkably occurs when stretch forming is carried out under a biaxial stress state to an austenitic steel sheet produced by the strip casting method, and is a ridge-like surface defect in which corrugations exist on the steel sheet surface in a direction parallel to the rolling direction and ridges having a predetermined angle to the rolling direction exist on the steel sheet surface. When the degree of press working is high, the maximum ridge height of this defect is as large as 2 to 6 .mu.m, and this is the critical defect which cannot be observed in the sheet produced by a conventional continuous casting/hot rolling/cold rolling process (hereinafter referred to as the "contentional method").
Work-surface roughening remarkably deteriorates the value of the strip-cast sheet products for working applications, and technology for preventing this defect has been necessary.